This invention relates to devices and methods for the quantitative determination of the concentration of a chemical constituent in a gaseous mixture. It has particular but not exclusive application to breath alcohol testing devices such as the ones sold by Intoximeters, Inc., 1901 Locust Street, St. Louis, Mo., under the trademark ALCO-SENSOR, and especially to such devices equipped with fuel cells constructed as described in Wolf U.S. Pat. Nos. 4,487,055 and 4,770,026, the disclosures of which are hereby incorporated by reference.
In breath alcohol testing devices presently used commercially, in which fuel cells are employed, the conventional way of determining breath alcohol is to measure a peak voltage across a resistor due to the flow of electrons obtained from the oxidation of breath alcohol on the surface of the fuel cell. There are a number of problems. The peaks become temporarily lower with repeated exposure to alcohol. The peaks also vary with temperature. In order to produce a high peak voltage, it is customary to put across the output terminals of the fuel cell a high external resistance, on the order of a thousand ohms, but the use of such a high resistance produces a voltage curve which goes to the peak and remains on a high plateau for an unacceptably long time. To overcome that problem, present systems provide for shorting the terminals, which drops the voltage to zero while the short is across the terminals. However, it is still necessary to let the cell recover, because if the short is removed in less than one-half to two minutes after the initial peak time, for example, the voltage creeps up. Peak values for the same concentration of alcohol decline with repeated use whether the terminals are shorted or not, and require 15-25 hours to recover to their original values.
Individual fuel cells differ in their characteristics. All of them slump with repeated exposure to alcohol in quick succession. Over time, their sensitivity decreases to a point at which they must be re-calibrated or replaced. Presently, the cell is replaced when it peaks too slowly, when it returns too slowly to a base line output, when the output at the peak declines beyond practical calibration, or when the background voltage begins creeping excessively after the short is removed from the cell terminals.
Wolf, U.S. Pat. No. 4,770,026, provides an apparatus and method that provides a measure of breath alcohol that is largely free of the drawbacks previously encountered with the use of fuel cells for this purpose. However, it remains dependent on the characteristic changes in a fuel cell's response curve caused by repeated exposure to alcohol and age. These changes increase the time required to perform an analysis and increase the time between successive analyses.
The present invention enhances the analytical capabilities of the device described in Wolf U.S. Pat. No. 4,770,026 by providing a new method for determining the level of breath alcohol or other gaseous constituent of a mixture. The improvement is applicable to a wide variety of other electronic analysis circuits associated with fuel cell detectors and to instruments for measuring a wide variety of reactive volatiles.
One of the objects of this invention is to reduce the time required for determining the level of breath alcohol or other reactive gases.
Another object is to reduce the computational requirements for such analysis.
Another object is to reduce the length of time required between successive such analyses.
Another object is to eliminate any error in a breath alcohol determination created by the residual effects of a previous test.
Other objects will become apparent to those skilled in the art in the light of the following description and accompanying drawing.